A color's hue may be best described as representing the color's dominant wavelength, which, for example, may be perceived by a person as red, green, blue, and so forth. To maximize the image quality of a display system, it may be important to maintain a color's hue constancy as the color's lightness (the color's luminous intensity or “luminance”) and/or chroma (the color's vividness or “chromaticity”) may be altered to provide brighter or dimmer images, for example. If the color's hue is allowed to change as the lightness and/or chroma are changed, there may be a shift in the color of the images. This may result in unnatural appearing images as well as constantly changing colors as image lightness and/or chroma are changed, for example.
Most modern display systems are additive in nature. One example is a digital micromirror device (DMD) based display system, wherein a large number of micromirrors arranged in an array pivot between two states (positions) to reflect colored light onto or away from a display plane. This means that as lightness and/or chroma are changed, the changes in the lightness and/or the chroma, respectively, may simply be added to or subtracted from the color.
However, with the human visual system, in order to maintain a color's perceived hue constancy with changing lightness and/or chroma, the color's numerical hue may also need to be changed. The degree to which a color's numerical hue needs to be changed may depend upon the amount of lightness and/or chroma being added to the color as well as the color and, potentially, the size of the color gamut. FIG. 1a illustrates an exemplary International Commission on Illumination (CIE) xy chromaticity diagram 100. The CIE xy chromaticity diagram 100 illustrates a visible spectrum 102 with constant hue curves for several colors, such as for red (curve 105), green (curve 110), and blue (curve 115). Also shown are constant hue curves for several other colors, such as yellow (curve 120), cyan (curve 125), and magenta (curve 130).
Examining the hue curve for the color red (curve 105), for example, in order to maintain perceived hue constancy, the numerical hue increases with increasing purity until about a mid-way point of the curve 105, then the numerical hue decreases with increasing purity until the curve 105 reaches the edge of the visible spectrum 102. The degree to which a color's hue must change to maintain hue constancy may depend on the color. For example, the constant perceived hue curve for the color yellow (curve 120) remains relatively linear with changing purity.
FIG. 1b illustrates an error that may exist between the hue curve for the color red (curve 105) and a line 155 of constant numerical hue for a purely additive display system. FIG. 1b displays a zoomed view of box labeled A shown in FIG. 1a. At starting points and end points for the curve 105 and the line 155, there may be very little or no difference between the hue curve 105 for the color red and the line 155 of constant hue for the color red. However, at about a midpoint of the curve 105, there may be a significant difference between the curve 105 and the line 155 in the color red, shown as highlight 160. Also shown is a second line 165, representing a line of constant numerical hue for the color yellow in an additive display system. A difference between the constant perceived hue curve for the color yellow (curve 120) and the second line 165 (the line of constant numerical hue for the color yellow) may be small, shown as highlight 170. Thus, the difference may be more visible when images that contain more of the color red than the color yellow are displayed.
The use of modern illumination systems, such as laser, LED, and other solid-state illumination, may result in a display system with a larger color gamut. The larger color gamut resulting from these illumination systems may exacerbate the difference between an additive display system's numerical hue constancy and the human visual system's perceived hue constancy, thereby potentially degrading the image quality of images being displayed.